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AE64 TELECOMMUNICATION SWITCHING SYSTEMS JUN 2015
© IETE 1
Solution Marks
Q.2 a. Discuss the various functions of a switching system.
Ans: Page No.56 &57 in text Book II
b.List out the advantages and disadvantage of strowger switching system
Ans: Advantages:
i) Inexpensive for small system
ii) Highly reliable due to the distributed nature of equipment.
Disadvantages:
1. As this switching involves heavy mechanical displacements, regular maintenance
by the skilled technicians is necessary.
2. It is not feasible to select an alternate route for interoffice calls, if all the trunks are
busy as the switching is by step through various selectors.
3. Step by step switching is limited to dial pulses. For touchtone telephones, special
devices have to be introduced between line finder and first selector to convert the tones
into dial pulses.
4. If calling rate is high, heavy operation is performed by the system and the life time
of the system is less.
5. The last two digits of the called line numbers are specifically determined by their
location on the connector. Congestion could arise when the switching system is heavily
loaded.
6. The capacity of switching system reduces if codes of different numbers are allotted
to various subscribers, such as fire service, police ambulance, fault regorts, directory
enquiry, operator assistance etc. In certain cases, the exchange capacity may be
reduced from 10000 to even 6000 customer lines.
7. The strowger system can accept only 7 to 9 pulses in 1 second. Hence if we dial fast,
the system cannot give correct performance.
Q.3 a. A group of 7 trunks is offered 4E of traffic, find (a) the grade of
service (b)the probability that only one trunk is busy (c) the
probability that only one trunk is free and (d) the probability that at
least one trunk is free.
Ans: N = 7, A = 4E
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b. Define loss system and delay system.
Ans: The type of system by which a blocked call is simply refused and is lost is
called loss system. In the second type of system, a blocked call remains in the system
and waits for a free line. This type of system is known as delay system.
c. Define Cent Call Seconds (CCS) and Grade of Service (GOS).
Ans: Cent call seconds:
It is also referred as hundred call seconds. CCS as a measure of traffic intensity is valid
only in telephone circuits. CCS represents a call time product. This is used as a
measure of the amount of traffic expressed in units of 100 seconds. Sometimes call
seconds (CS) and call minutes (CM) are also used as a measure of traffic intensity. The
relation between erlang and CCS is given by
1E = 36 CCS = 3600 CS = 60 CM
Grade of Service (GOS):
For non-blocking service of an exchange, it is necessary to provide as many lines as
there are subscribers. But it is not economical. So, some calls have to be rejected and
retried when the lines are being used by other subscribers. The grade of service refers
to the proportion of unsuccessful calls relative to the total number of calls. GOS is
defined as the ratio of lost traffic to offered traffic.
GOS =Blocked Busy Hour calls/Offered Busy Hour calls
Q.4 a. A three stage switching structure supports 100 inlets and 400
outlets. Find
(i) the number of cross points (ii) the number of primary and
secondary switches used in the design.
Ans: We know that
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m = M/M+N and n = N/M+N
m = 100
1. if m=5, n=20, there are :
20 primary switches of size 5 x 5
5 secondary switches of size 20 x 20
20 tertiary switches of size 5 x 20
2. if m = 4, n =16, there are:
25 primary switches of size 4 x 4
4 secondary switches of size 25 x 25
25 tertiary switches of size 4 x 16
b. What are single stage and multistage switching networks? Compare the
strengths and weaknesses of each.
Sr.
No.
Single stage Multistage
1 Inlet to outlet connection is through a
single cross point.
Inlet to Outlet connection is
through
multiple cross points
2 Use of single cross point per connection
results in better quality link.
Use of multiple cross points
may degrade the quality of a
connection.
3 Each individual cross point can be used
for only one inlet/outlet pair connection.
Same cross point can be used
establish connection between
a number of inlet/outlet pairs.
4 A specific cross point is needed for each
specific connection.
A specific connection may be
established by using sets of
cross points.
5 If a cross points fails, associated
connection cannot be establish-
There is no redundancy.
Alternative cross-points and
paths are available.
6 Cross points are inefficiently used. Only
one cross point in each row or column of a
square or triangular switch matrix is even
in use, even if all the lines are active.
Cross points are used
Efficiently
7 Number of cross points is
Prohibitive
Number of cross points
is reduced significantly
8 The network is non blocking
in character
The network is blocking
in character
Q.5 a. Draw and explain Space Division Switching in detail.
Ans: Space Switches:
Connections can be made between incoming and outgoing PCM highways by means of
a cross point matrix of the form shown in Fig. However, different channels of an
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incoming PCM frame may need to be switched by different cross points in order to
reach different destinations. The cross point is therefore a two-input AND gate. One
input is connected to the incoming PCM highway and the other to a connection store
that produce a pulse at the required instant. A group of cross points gates can be
implemented as an integrated circuit, for example by using a multiplexer chip.
Fig. shows a space switch with k incoming and m outgoing PCM highways, each
carrying n channels. The connections store for each column of cross points is a
memory with an address location for each time-slot, which stores the number of the
cross points to be operated in that time slot. This number is written into the address by
the controlling processor in order to setup the connection. The numbers are read out
cyclically, in synchronism with the incoming PCM frame. In each time slot, the
number stored at the corresponding store address is read out and decoding logic
converts this into a pulse or a single lead to operate the relevant cross point.
Since a cross point can make a different connection in each of the n time-slots, it is
equivalent to n cross points in a space division network. The complete space switch is
thus equivalent to n separate k x m switches in a space division switching network.
b. Explain and compare T-S-T and S-T-S switching techniques.
Ans: STS Switching.
In STS switching, the time stage is sandwiched between two space arrays. The digital
switching system ITS 4/5 of USA (1976) uses the STS switching configuration. It
handles 3000 trunks and accommodates 1500 Erlangs of traffic. Fig. shows the space
time- space (S-T-S) switching network for M incoming and outgoing PCM highways.
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Establishing a path through an STS switch requires finding a time switch array with an
available unit’s access during the incoming time slot and an available read access
during the desired outgoing time slot. The input side space stage as well as the output
side space stage is free to utilise any free time switch modules. In the diagram shown in
Fig the time slot 2 is connected to the TSM 2 where the time slot allotted is 16 and
passed to the (M – 1)th line of output space array. Thus the path is provided. This
structure is of non-blocking nature.
TST Switching:
In TST switching the space stage is sandwiched between two time stage switches. Of
all the multistage switching, TST is a popular one. The principle of operation of TST
switching is shown in Fig. In figure, two flows of time slots, one for each direction are
connected together.
The information arriving at the incoming link of TDM channel is delayed in the· inlet
times stage until an appropriate path through the space stage is available. Then the
information is transferred through the space stage to the appropriate outlet time stage.
Here the information is held until the desired outgoing time slot occurs. Any space
stage time slot can be used to establish a connection. The space stage operates in a time
divided fashion, independently of the external TDM links. There are many alternative
paths between a prescribed input and output unlike a two stage network which has only
one fixed path.
Q.6 a. Define Stored Program Control (SPC) and explain the distributed
SPC in detail with the help of a diagram.
Ans: In stored program control systems, a program or set of instructions to the
computer is stored in its memory and the instructions are executed automatically one
by one by the processor. Carrying out the exchange control functions through programs
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stored in the memory of a computer led to this name.
Distributed SPC:
The introduction of distributed SPC enabled customers to be provided with a wider
range of services than those available with centralised and electromechanical switching
system. Instead of all processing being performed by a one or two processor in
centralised switching, functions are delegated to separate small processors (referred as
regional processors). But central processors are still required to direct the regional
processors and to perform more complex tasks. The distributed SPC offers better
availability and reliability than the centralised SPC. Entire exchange control functions
may be decomposed either horizontally or vertically for distributed processing. In
vertical decomposition, the exchange environment is divided into several blocks and
each block is assigned to a processor that performs all control functions related to that
block of equipments. In horizontal decomposition, each processor performs one or
some of the exchange control functions. Figure shows the distributed control where
switching equipment is divided into parts, each of which has its own processor.
b. Enlist & explain the various steps involved in processing a call.
Ans:
1. Idle state. At this state, the subscriber handset is in ‘on-hook’ condition. The
exchange is ready to detect the call request from the subscriber.
2. Call request identification. The exchange identifies a line requiring for a service.
When the handset is lifted, current flows in the line called seize signal indicates the call
request.
3. Providing dial tone. Once the seize signal is received, an exchange sends a dial tone
to the calling subscriber to dial the numbers.
4. Address analysis. Once the first digit received, the exchange removes the dial tone
and collects all numbers. Then the address is analysed for the validity of the number,
local, STD or ISD etc. If the number is invalid, a recorded message may be sent to the
calling subscriber and terminates call request.
5. Called line identification. The exchange determines the required outgoing line
termination from the address that it has received.
6. Status of called subscriber. The called line may be busy or free or unavailable or
even out of service. In the case of PBX, where the customer have a group of lines, the
exchange tests each termination until either it finds a free one or all one found busy.
For busy, number unobtainable or the handset off hook, a status signal or call progress
signal is sent to the calling subscribers for line termination. Now the exchange resumes
idle state.
7. Ringing. Once, the exchange finds the called subscriber is free, power ringing is
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provided to the called subscriber and audible ringing to the calling subscriber.
8. Path setup. When the called subscriber lifts his handset, the line is looped and
ringing is removed. Once the conversation started, the exchange completes the
connections between the subscribers.
9. Supervision. The exchange supervises the connection to detect the end of the call
for charging.
10. Clear signal. Once the need for connection is over, either customer may replace his
handset. It causes the line current seize and provides a clear signal to exchange. If the
calling subscriber replaces his phone set, the clear signal sent to the exchange is called
clear forward signal. If called subscriber do first, the clear signal is called clear
backward signal.
Q.7 a. Describe the architecture of SS7 common channel signalling
network with the help of a neat labelled diagram.
Ans: A block schematic diagram of the CCITT no. 7 signalling system is shown in
fig. Signal messages are passed from the central processor of the sending exchange to
the CCS system. This consists of the microprocessor based subsystem. The signalling
control subsystems, the signalling termination subsystem and the error control
subsystem. The signalling control subsystem structures the messages in the appropriate
format and queues them for transmission. When there are no messages to send, it
generates filler messages to keep the link active. Messages then passed to the signalling
termination sub system, where complete signal units (SU) are assembled using
sequence numbers and check bits generated by the error control subsystem. At the
receiving terminal, the reverse sequence is carried out. The levels are as follows:
Level 1: The Physical Layer
Level 2: The Data Link Level
Level 3: The signalling network level
Level 4: The User Part
The relationship between these levels and the layers of the OSI model is shown in Fig.
The user part encompasses layers 4 to 7 of the OSI model. Level 1 is the means of
sending bit streams over a physical path. It uses times lot 16 of a 2 Mbit/s PCM system
or times slot 24 of a 1.5 M bit/s system. Level 2 performs the functions of error control,
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link initialization, error rate monitoring, flow control and delineation of messages.
Level 3 provides the functions required for a signalling network. Each node in the
network has a single point odd, which is a 14 bit address. Every message contains a
point code of the originating and terminating nodes for that messages. Levels 1 to 3
form the message transfer part (MTP) of CCITT no. 7. Level 4 is the user part. This
consists of the processes for handling the service being supported by the signalling
system. The message transfer part is capable of supporting many different user parts.
So far, three have been defined: the telephone user part(TUE), the data user part (DUP)
and the (ISDN) user part (ISDN-UP).
b. Explain architecture of T1 link and DS-1 frame format in relation to
the PCM signalling
Ans:
In PCM systems, signalling and speech are sampled, coded and transmitted within the
frame of PCM channels. Thus, with PCM, a convenient way of transmission is
possible. The signalling information and speech information carried in the same time
slot is referred as inslot signalling. The signalling information carried in a separate time
slot is referred as outslot signalling. The timeslot of the inslot signalling is fixed at
eight bits. As one bit is used for signalling, the speech bit rate is reduced to 56 kbps
from 64 kbps and the bandwidth is reduced. Telephone channels are combined by time
division multiplexing to form an assembly of 24 or 30 channels. This is known as
primary multiplex group.
DS1 24 channel system.
DS0 is a 64-kbps signal that makes up the basis for the DS1. Twenty four DS0s
combined to produce DS1. Fig shows the architecture of a T1 link and DS-1 frame
format.
Incoming analog signals were time division multiplexed and digitized for transmission.
Each individual TDM channel is assigned 8 bits per time slot. Each frame is made of
24 × 8 bits = 192 bits plus one additional bit added to each frame to identity the fame
boundaries. Thus each frame contains 193 bits. The frame interval is 125 micro sec.
Hence the basic T1 line rate is 1.544 Mbps. This line rate has been established as the
fundamental standard for digital transmission.
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Q.8 a. Explain and compare bus and ring topology used in LAN
technology.
Ans: Bus Topology:
This topology shares a single link or path way among all users. This common single
path way is known as bus. In this topology, the link serves as a high way for all data
signals, and users connect on to the bus at their node location. In bus configurations,
network control is not centralized to a particular node. Here control is distributed
among all nodes connected to the LAN. Data transmission on a bus network is usually
in the form of small packets containing user addresses and data. When one node/user
desires to transmit data to another station, it monitors the bus to determine if it is
currently being used. If no other nodes/users are communicating over the network, the
monitoring node/user can start to transmit its data. Each node must monitor all
transmission on the network and determine which are intended for them.
Ring Topology:
In ring topology, all user nodes are connected with the physical path acting as links of a
chain and the last user node is connected back to the first node. A signal going on to
the next node must be processed by the first node, which then passes it through to the
next node. Adding a new user requires breaking the ring temporarily, inserting the new
node and then re-establishing the complete ring path.
b. Explain the advantages and services offered by Asynchronous
Transfer Mode (ATM)
Ans:
The most important advantages or benefits of the ATM are:
1. A much wider array of information can be transmitted using ATM technology, for
example, voice, data, images, CATV scans, MRI images and video conferencing.
2. ATM delivers bandwidth on demand, is not dependent on applications and works at
a data rate from 1.5 Mbps to 2 Gbps.
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3. All types of networking, from LANs to WANs and from backbone to desktop can be
integrated by ATM technology.
4. The service is connection oriented, with data transferred over a virtual circuit.
5. ATM switches are statistical multiplexing.
6. Higher quality of service.
7. Wider array of information can be handled.
8. Accepts variety of transmission media such as optical fiber or twisted pair cable.
9. Works with current LAN and WAN technologies and supports current protocols
such as TCP/IP.
ATM Services.
ATM forum specifies five types of services. They are:
1. Constant bit rate (CBR). This is used for emulating circuit switching. The cell rate
is constant with time. Telephone traffic, videoconferencing and television are the
examples that use CBR.
2. Variable bit rate–non real time (VBR–NRT). This service allows users to send
traffic at a rate that varies with time depending on the availability of user information.
Multimedia email is an example of VBR–NRT.
3. Variable bit rate–real time (VBR–RT). This service is similar to VBR–NRT, but it
is designed for applications that are sensitive to cell delay variation. Examples for real
time VBR are voice with speech activity detection (SAD) and interactive compressed
video.
4. Available bit rate (ABR). This service provides rate based flow control and is
aimed at data traffic such as file transfer and e-mail.
5. Unspecified bit rate (UBR). This class is widely used today for TCP/IP.
Q.9 a. Explain Primary Rate Interface (PRI) and Basic Rate Interface
(BRI) of ISDN.
Ans:
Basic Rate Interface (BRI):
BRI is made up of two B-channels (Bearer channels) and one D channel. Therefore the
total rate is 2B + D. B channels are 64 kbps and can be used for voice and data
communications. The D channel is 16 kbps and is used for call initialization and
signalling connections. Fig. 12.2 shows ISDN BRI. BRI is the most appropriate type of
ISDN service
for computer connections and individual use. Of three digital channels (2B + D), of
each of the channels can be used simultaneously. Thus a subscriber can perform several
communications tasks at the same time. BRI is designed to carry the most data possible
to the home through existing copper phone lines.
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Primary Rate Interface (PRI):
PRI in North America has 23 B channels and one 64 K D channel or the total rate is 23
B + 1D, having a total bandwidth 1.544 Mbps. (including 8 kbps of overhead) PRI in
rest of the world uses 30 B channels and one D channel
or 30 B + D with total rate of 2.048 Mbps. The number of B channels is limited by the
size of the standard trunk line used in the region. Fig shows the ISDN PRI interface.
Unlike BRI, PRI does not support a bus configuration and only one device can be
connected to a PRI line. A PBX, however can reallocate ISDN PRI resources on to
multiple BRI buses. Fig shows the ISDN PRI concept. A single PRI connection is
usually much less expensive than obtaining the equivalent number of B channels
through multiple BRI connections. The 1.544 Mbps of a PRI can be divided up in
many ways to meet the requirements of many users. This indicates combination of 64
kbps B channels or all capacity of B channels (In LAN to LAN case) can be used.
b. Write short notes on the following:
(i) B-ISDN
(ii) Alternative routing
Ans: B-ISDN
BISDN Configuration: Fig. shows how access to the BISDN network is
accomplished. Each peripheral device is interfaced to the access node of a BISDN
network through a broadband distant terminal (BDT). The BDT is responsible for
electrical to optical conversion, multiplexing of peripherals, and maintenance of the
subscriber’s local system. Excess nodes concentrate several BDT’s into high speed
optical fiber line directed through a feeder point into a service node. Most of the
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control function for system excess is managed by the service node, such as call
processing, administrative function and switching and maintenance functions. The
functional modules are interconnected in a star configuration and include switching,
administrative, gateway, and maintenance modules. The interconnection of the function
module is shown in Fig. The central control hub acts as the end user interface for
control signalling and data traffic maintenance. In essence, it oversees the operation of
the modules.
Subscriber terminal near the control office may by pass the excess nodes entirely and
the directly connected to the BISDN network through a service node. BISDN nodes
that used optical fiber cables can utilize much wider band width and consequently,
have higher transmission rates and offer more channel handling capacity than ISDN
systems.
ii) Alternative Routing:
Based on the assumption that the routing is made only by direct routing or tandom
routing, it is found that to route a stream of traffic, tandom route is more economical.
In fact, even greater economics are often possible if just a proportion of the traffic is
routed directly. This approach is known as alternative routing. In alternative routing,
connections should use the direct trunks (referred as high usage route), because direct
route provides better transmission quality and use fewer network facilities. If all the
direct trunks are busy, calls are routed via a tandom exchanges or alternate routes to
maintain suitably low blocking probabilities. Thus, the networks are designed to
allocate a limited number of heavily utilized trunks in the direct route and provide
alternate routes for over flow.
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TEXT BOOK
Telecommunications Switching, Traffic and Networks, J.E.Flood, Pearson Education- 2006
Telecommunication Switching Systems and Networks, Thiagarajan Viswanathan, Prentice Hall of India Pvt. Ltd, 2007
Digital Telephony, John C Bellamy, John Wiley (International Student Edition)